The present invention relates generally to operations performed in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides methods and apparatus useful in gravel packing operations.
Frequently, a horizontal or highly deviated portion of a wellbore is completed without being lined with protective casing and cement. If the wellbore portion intersects an unconsolidated or very low strength formation from which it is desired to produce fluids, it may be desirable to perform a completion operation known as gravel packing. In a gravel packing operation, sand or other particulate material is flowed into an annular space formed radially between the wellbore and one or more screens attached to a special purpose packer set in the wellbore. The sand and screens act to prevent the formation from breaking up and flowing to the earth's surface along with the fluids produced from the formation.
In some horizontal or highly deviated uncased wellbore completions, a "filter cake" is applied to the walls of the wellbore to aid in stabilizing the formation intersected by the wellbore. The filter cake temporarily prevents breaking up of the formation or, ultimately, collapse of the formation during completion operations. In some cases, the filter cake may be a gelatinous material spotted across the uncased wellbore, or it may be material conveyed to the uncased wellbore by mud circulation, etc.
In order for the filter cake to provide maximum stabilization of the formation it is generally desirable for positive pressure to be applied from the wellbore to the formation. That is, fluid pressure in the wellbore should exceed fluid pressure in the formation by a desired amount. This positive pressure acts, in essence, to press the filter cake against the formation. Thus, although the filter cake is not generally pressure-tight, if a positive pressure is continuously applied to the filter cake, the filter cake will provide adequate support to prevent damage to the formation.
Unfortunately, conventional gravel packing operations do not permit a positive pressure to be continuously applied to the filter cake. Each of these operations, therefore, runs the risk that the formation will become sufficiently destabilized during the operation to cause damage to the formation. This may result in the operation being aborted, equipment being caught in a collapsed wellbore, etc., each of which would require great time and expense to remedy.
Therefore, it would be quite desirable to provide methods and apparatus for completing a subterranean well which permit continuous application of positive pressure to a wellbore. Such methods and apparatus would be particularly desirable in gravel packing operations performed in uncased portions of horizontal or highly deviated wellbores intersecting unconsolidated or very low strength formations in which filter cakes are utilized to stabilize the formations, although the methods and apparatus would be quite useful in other operations as well.